Crude oil production from oil in subterranean reservoirs may involve use of various flooding methods as the natural forces, which are used in the “primary recovery” process, become depleted. A large portion of the crude oil may have to be driven out of the formation in “secondary” or “tertiary” recovery processes. In addition, some reservoirs may not have sufficient natural forces for oil production even by primary recovery processes. The production of crude oil using such flooding methods is one example of enhanced oil recovery process.
Currently, the petroleum industry is re-evaluating technologies that will improve the ability to recover remaining and untapped oil from the subterranean reservoirs. Injecting a displacing fluid or gas may begin early, long before the complete depletion of the field by primary recovery processes. Methods for improving displacement efficiency or sweep efficiency may be used at the very beginning of the first injection of a displacing fluid or gas, rather than under secondary and tertiary recovery conditions.
The easiest method of flooding a subterranean reservoir for the production of crude oil is by injecting a liquid or a gas into the well to force the oil to the surface. Water flooding is the most widely used fluid. However, water does not readily displace oil because of the high interfacial tension between the two liquids which result in high capillary pressure that trap in porous media.
The addition of chemicals to modify the properties of the flooding liquid is well known in the art of improved/enhanced oil recovery. Surfactants are one class of chemical compounds that have been used in aqueous media for enhanced oil recovery. Surfactants have been found to effectively lower the interfacial tension between oil and water and enable mobilization of trapped oil through the reservoir.
Alkylaryl sulfonates have been used as surfactants for enhanced oil recovery. They have been used in surfactant flooding, alone, or in conjunction with co surfactants and/or sacrificial agents. Alkylaryl sulfonates are generally used not only because they are able to lower the interfacial tension between oil and water, but also because when used in conjunction with varying amounts of other salts, such as, sodium chloride they exhibit desirable phase behavior. Depending on the molecular weight and molecular weight distribution, branching and point of attachment of the aryl group to the alkyl groups, alkylaryl sulfonates can be tailored to preferentially reside in the aqueous or oleic phases at different electrolyte concentrations, i.e., salinities. At low salinities the alkylaryl sulfonates reside in water and at high salinities they partition into the oil. In either case, the swollen micellar solutions that contain surfactants, oil and water are termed microemulsions. At optimal salinity an equal volume of oil and water are solubilized in the microemulsion. For well tailored and matched alkylaryl sulfonates, the high volumes of oil and water solubilized in the microemulsion result in ultra-low interfacial tensions that provide potential for high oil recovery from reservoirs.
The salinity of the water in subterranean hydrocarbon reservoirs may vary a great deal, for example, the Minas oil field in Indonesia has total dissolved salts of between 0.2 and 0.3 weight percent. Other reservoirs may have salinities as high as or higher than 2.0 percent sodium chloride and over 0.5 percent calcium chloride and magnesium chloride. It is desirable to optimize the alkylaryl sulfonates for surfactant flooding for enhanced oil recovery for a particular reservoir by evaluating tailored versions of the alkylaryl sulfonates with native reservoir brine and reservoir oil under ambient reservoir conditions via phase behavior experiments. In addition to the phase behavior experiments a few interfacial tension measurements are needed to verify that the interfacial tensions are acceptably low. In addition to testing the surfactants with native reservoir brines additional tests with injected solutions are needed as in some instances the injectate brine is different from native reservoir brines.
Generally, pure alkylaryl sulfonates, that is, those having a narrow range of molecular weights, are useful for recovery of light crude oils. Such alkylaryl sulfonates have exhibited poor phase behavior, i.e., poor potential to recover oils, containing high wax content. Oils with typically high wax content generally have high equivalent average carbon numbers (EACN's). The equivalent alkane carbon number (EACN) is a representation of an average carbon chain length of a hydrocarbon mixture. As an illustration, pentane, hexane and heptane have alkane carbon numbers of 5, 6 and 7 respectively. However a mixture containing 1 mole of pentane and one mole of hexane would have an EACN of 5.5. Field crude oils are complex mixtures but when interacting with surfactants, they behave as a single component fluid with an EACN that is a mole fraction average of its constituents.
Alkylaryl sulfonates having a broad spectrum of carbon chain lengths in the alkyl group are more desirable for use to recover waxy crude oils or crude oils with high equivalent average carbon numbers (EACN's). In addition to optimizing the molecular weight and/or molecular weight distribution of an alkylaryl sulfonate to maximize the amount of oil in the aforementioned micro-emulsion, the use of other components in combination with the alkylarylsulfonate, such as inorganic salts, co-solvents, polymeric materials and co-surfactants may improve phase behavior. The performance of an enhanced oil recovery formulation may also be measured by the oil solubilization parameter, which the volume of oil dissolved per unit volume of surfactant. The oil solubilization is inversely proportional to the interfacial tensions. In addition the performance is also measure by the ability of the formulation to achieve stable microemulsions and low interfacial tensions rapidly, i.e., in less than one day in the laboratory.
A number of patents and patent applications have discussed methods for enhanced oil recovery using surfactant flooding. In addition to the use of surfactants, there are a number of patent and patent applications discussing the use of co-surfactants and sacrificial agents for enhanced oil recovery.
Hsu et al., U.S. Pat. No. 6,022,843 discloses an improved concentrated surfactant formulation and process for the recovery of residual oil from subterranean petroleum reservoirs, and more particularly an improved alkali surfactant flooding process which results in ultra-low interfacial tensions between the injected material and the residual oil, wherein the concentrated surfactant formulation is supplied at a concentration above, at, or, below its critical micelle concentration, also providing in situ formation of surface active material formed from the reaction of naturally occurring organic acidic components with the injected alkali material which serves to increase the efficiency of oil recovery.
Berger et al., U.S. Published Patent Application No. 2005/0199395A1 discloses an oil recovery process and a particular class of alkylaryl sulfonate surfactants. The surfactants are derived from an alpha-olefin stream having a broad distribution of even carbon number ranging from 12 to 28 or more.
A general treatise on enhanced oil recovery is Basic Concepts in Enhanced Oil Recovery Processes edited by M. Baviere (published for SCI by Elsevier Applied Science, London and New York, 1991).
Prior surfactant-polymer solutions used for enhanced oil recovery were historically hazy and/or translucent. The solutions were hazy because of large micellar size. The surfactants are most effective when they have a balanced solubility in oil and water. In most cases the surfactant and its micelles are only sparingly soluble in the aqueous phase; and therefore, they exist more as a dispersion of relatively large micelles. The size of these micelles approaches the wavelengths of light and thus, interferes with the passage of light and makes the solutions hazy or opaque.
When surfactant flooding is applied for oil recovery, the surfactant/polymer formulations may need to propagate for distances on the order of 100-600 ft. Over such distances these hazy solutions tend to phase separate and also get filtered out by the porous medium.
In the past minimal attention has been paid to aqueous stability. During the London floods, another operator achieved aqueous stability of the surfactant-polymer formulation, but did not consider the interactions with the chase solutions. Similarly, during the surfactant field trial in Minas in 2002, there was not an importance placed upon aqueous stability, which was a contributing factor to lower than anticipated oil recovery.